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Introduction to Petrography

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Earth Science 232
Petrography
Course notes by 
Shaun Frape and Alec Blyth
Winter 2002
1
Petrology - Introduction
Some Definitions
Petra ⇒ Greek for “rock”
Logos ⇒ Greek for “disclosure or explanation”
Petrology -The branch of geology dealing with the 
origin, occurrence, structure, and history of 
rocks, especially igneous and sedimentary 
rocks.
Igneous rock: a rock that solidifies from molten or 
partially molten material (ie magma).
Sedimentary rock: a rock that results from consolidation 
of loose sediment or chemicals precipitating from 
solution at or near the earth’s surface; or organic rock 
consisting of the secretions or remains of plants and 
animals.
Metamorphic rock: rock derived from pre-existing rocks 
by mineralogical, chemical or structural changes 
(especially in the solid-state).
Note: Some borderline situations and rock types do 
exist. They are not common compared to the overall 
percentage of rocks in existence eg (1) volcanic tuffs, 
igneous or sedimentary, (2) serpentinite Mg3Si2O5(OH)4
from low T seafloor alteration, igneous or metamorphic.
2
Classification can be aided using Table 1-1
(1) Outcrop Characteristics
(2) Textures (general)
(3) Examples of Minerals
3
Abundance of Different Rock Types
There are several schemes and classifications.
(1) The abundance of the 3 rock groups on the 
continents
Sedimentary ~66%
Igneous + Metamorphic ~34% (bulk is igneous)
(2) If we consider the ocean sediments then some of the 
schemes have sedimentary rocks as high as 80%.
(3) Note on Figure 1-1 the relationship with time. As 
you may have learned in Stratigraphy, there are very 
large differences in time due to erosion or non-
deposition.
4
Igneous Petrology
Petrology - is one of the key courses for a geologist
- you must be able to interpret the rocks 
crystallization and chemical history and 
therefore have to familiarize yourself with the 
“Tools of Petrology”.
Tools - [A] Petrography
Petrography is the microscopic identification and 
interrelationships of mineral grains in the fabric of a 
rock.
Eg: When you finish studying igneous rocks this term 
you will develop the feeling that most hand specimens 
and slides are made up of 2 basic types of mineral 
assemblages.
1) Quartz, K-Feldspars, Na-Plagioclase and Micas or
2) Ca-Plagioclase, Pyroxenes, Amphiboles
The former are usually light in colour while the latter are 
dark.
5
Tools - [B] Phase Diagrams
Igneous Rocks are rocks that solidify from molten or 
partially molten material ie, from magma.
Therefore, a very important tool is experimental and 
observational High Temperature Geochemistry and the 
use of Phase Diagrams.
In this course we will examine most of the major 
igneous rock forming mineral assemblages using these 
two tools.
As a brief start, lets examine something you should be 
familiar with.
In the 1920’s N.L. Bowen noted that under a variety of 
changing temperature conditions, a set group of 
minerals always crystallized in the same order from a 
melt. (P50-54 Jackson)
6
What is important here is to look at the details and 
interrelationships of the mineral families.
Eg: (a) Why are Olivine and the Plagioclase 
Group of minerals similar in behavior [Solid 
Solution Series]?
(b) Why are they Different?
We will examine each of the systems, examining the 
controls to the formation of mineral phases
Magma Differentiation
(1) Fractional Crystallization
The Role of Gravity and Density
(a) Basaltic magma has density of 2.7 g/cm3 while 
early crystals of Olivine or Pyroxene are 3.3 g/cm3
(i) In large chambers these sink and are 
removed
(ii) Magma becomes lighter; chemical 
components are missing
(iii) Light components like leucite (2.5 
g/cm3) float
7
Magma Differentiation - cont.
(b) Magma changes composition with crystallization 
and individual mineral phases change composition 
with time as early ingredients are used up. We use 
binary phase diagrams to trace these changes
(2) Filter Differentiation or Filter Press Action
Flowing magma enters fractures. Crystals are blocked 
while the crystal-free magma continues. Crystals 
settling rearrange and tend to expel free liquid (filter 
press). Less dense felsic/granitic magmas often 
separate and float to the top of the chamber, cool and 
form a cap.
(3) Flow Differentiation
In flowing moving pipes, early formed crystals collect 
and orient in the middle or center of the pipe- like 
stirring beaker.
(4) Zoning
Incomplete reactions - lack of diffusion. Cores or 
crystals hive higher temperature or different 
compositions than the rims. Use up certain ingredients 
first, then can only make residual minerals.
8
Magma Differentiation - cont.
(5) Liquid Immiscibility - eg Oil & Water
Cool a magma and some ingredients separate 
eg - sulphide melt
-carbonatites separate from mafic alkalines
- felsic blobs in ultramafics
Immiscibility appears to need :
high K; low H2O; P under 4 Kbars; 
< 55 % SiO2 and high Si/Al
(6) Volatiles
Late-stage differentiation mechanism by gaseous 
transfer, secondary boiling (flashing). If gas rises it 
changes the residual melt composition. Some 
elements concentrate in the gas phase (eg: Na, Cl, F, 
Mn, Ti, Rb, La, Ce, Y, Zr, U) and will enrich upward 
along with SiO2 in the magma chamber. Others 
elements prefer the melt (eg: Ca, Mg, Cr, Fe, P,K,B) 
and will be enriched downward in the chamber. 
Eg: Na depletion due to gas stripping in deeper 
Andesitic magmas. We can analyze elements and use 
this to tell magma depth conditions.
9
Magma Differentiation - cont.
(7) Assimilation
- Melting and fractional crystallization of host rocks.
- Contaminants
(8) Magma mixing
- Diffusional process
10